In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more radio frequency (RF) carriers each defining one or more ranges of frequency spectrum and having a respective “downlink channel” for carrying communications from the base station to UEs and a respective “uplink channel” for carrying communications from the UEs to the base station. Such carriers may be frequency division duplex (FDD), in which the downlink and uplink channels are defined as a pair of separate respective ranges of frequency, or time division duplex (TDD), in which the downlink and uplink channels are defined on a common range of frequency but distinguished from each other through time division multiplexing. Further, the downlink channel and uplink channel of each carrier may also be divided into respective sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS) for example, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are each divided over their frequency bandwidth into sub-carriers that are grouped within each slot or sub-frame into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station on a particular carrier on which the base station is configured to provide, and the base station may then serve the UE on that carrier, scheduling particular downlink and uplink resource blocks on that carrier to carry data communications to and from the UE. Further, the base station and UE may modulate their air interface data communications at a coding rate selected based on quality of the UE's coverage, such as with higher rate coding rate when the UE is in better coverage of the base station and with a lower coding rate when the UE is in worse coverage of the base station.
In such an LTE system, for instance, when the base station has data to transmit to a UE, the base station may assign certain downlink resource blocks (and thus certain sub-carriers) in a given sub-frame for use to carry the data to the UE at a particular coding rate, and the base station may then (i) transmit to the UE in that sub-frame a downlink control information (DCI) message that specifies the assigned resource blocks and coding rate and (ii) encode and transmit the data to the UE in the assigned resource blocks in that sub-frame. Per the DCI message, the UE would then read the transmitted data from the assigned resource blocks. Likewise, when the UE has data to transmit to the base station and accordingly transmits a scheduling request to the base station, the base station may assign certain uplink resource blocks in a given sub-frame for use to carry the data from the UE at a particular coding rate and may transmit to the UE, in advance of that sub-frame, a DCI message that specifies the assigned resource blocks and coding rate. And the UE may then encode and transmit the data to the base station in the assigned resource blocks in that sub-frame.
With such an arrangement, the bandwidth of the carrier on which the base station serves a UE may pose an effective limit on the peak rate of data communication between the base station and the UE, as the bandwidth would define only a limited number of resource blocks per slot or sub-frame, with data rate per resource block being further limited based on air interface conditions.
One way to help overcome this per-carrier data rate limitation is to have a base station serve a UE on multiple carriers at once, providing what is known as “carrier aggregation” service. With carrier aggregation service, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands are aggregated together as “component carriers” to increase the overall bandwidth available per slot by providing a greater extent of air interface resources in which the base station can schedule uplink and downlink communication.
With carrier aggregation, one of the carriers may be deemed a primary component carrier or primary cell (PCell) on which the base station serves the UE, and each other carrier may be deemed to be a secondary component carrier or secondary cell (SCell) on which the base station serves the UE. In particular, the PCell may be the carrier on which the UE is attached with the serving base station and may thus carry control signaling (such as scheduling requests and DCI messages) between the base station and the UE, in addition to carrying scheduled data transmissions between the base station and the UE. Each added SCell may then function to increase the total bandwidth on which the base station serves the UE with scheduled data transmissions.
Thus, in an example arrangement, if a base station is serving a UE with carrier aggregation on a PCell and one SCell, the base station may assign to the UE in a given sub-frame certain downlink resource blocks on the PCell and certain downlink resource blocks on the SCell, and the base station may (i) transmit to the UE in that sub-frame on the PCell a DCI message that specifies all of those assigned resource blocks and (ii) encode and transmit the data to the UE in the assigned resource blocks on the PCell and SCell. Likewise, when the UE has data to transmit to the base station and transmits a scheduling request to the base station, the base station may assign in a given sub-frame certain uplink resource blocks on the PCell and certain uplink resource blocks on the SCell and may transmit to the UE on the PCell, in advance of that sub-frame, a DCI message that specifies all of those assigned resource blocks. The UE may then encode and transmit the data to the base station in the assigned resource blocks on the PCell and SCell. (In an alternative arrangement, the base station may make use of the SCell just for its added downlink bandwidth, not for its added uplink bandwidth, thus effectively providing just downlink carrier aggregation.)